High-integrity, high-performance aerospace structures are often fabricated by overlaying sheets of metallic or non-metallic materials and fastening the sheets to one another with fasteners, such as rivets or bolts. A series of connected sheets may be fastened together to form complex structures such as aircraft fuselages and fuel tanks.
When the sheets of material are fastened together, the surfaces of the sheets are placed in intimate contact with one another and become, for all intents and purposes, invisible without any discernable boundary. These surfaces are known as “faying surfaces”. The respective faying surfaces must intimately mate with one another in order to provide a strong physical connection or bond between the sheets of material. The conformance of the faying surfaces is even more important if the resultant fabricated structure is to contain volumes of liquid or gas. For instance, the faying surfaces of an aircraft fuselage must prevent the escape of air from a pressurized cabin, and the faying surfaces of an aircraft fuel tank must prevent the leakage of fuel.
Sealant materials are often applied to faying surfaces and to fasteners disposed through the faying surfaces to provide improved sealing and impermeability to liquids and gases contained within the fabricated and assembled structures. Fay-surface sealants are retained in a compressed state between the faying surfaces of the joined structural components and must not flow or squeeze out from between the mating fay-surfaces during the service life of the assembled article. Still, in these situations the composition must show sufficient compliance or flow characteristics to adequately effect a seal between the two mating fay surfaces, particularly if those surfaces possess slight irregularities or are otherwise not perfectly conforming.
Liquid or wet polysulfide resins are the most used faying-surface sealant materials because of their favorable chemical and physical properties, their ability to be pigmented, and their acceptance as an effective and efficient sealant system for use in the aircraft industry. However, since these sealant materials are applied in a wet, viscous state, the coated objects are difficult to handle after having the wet liquid polymer resins applied to them. Furthermore, the polysulfide sealant materials tend to degrade once in contact with high sulfur fuels typically used in aerospace applications.
Several alternatives to wet polysulfide sealant materials have been proposed over the years. Many of these alternatives use dry application processes and avoid the need for complicated wet applications. Nitrile-phenolic-based, thin-film adhesives provide for improved fuel tank sealing performance over the conventional wet, polysulfide sealing method. Also, sealant materials including fluoroelastomers, fluorosilicones, polyesters, polythioethers, polyurethanes, and polyureas have been developed. In addition, many technological advances in corrosion-inhibiting pigments, greatly reduced time and temperature curing parameters, elimination of fastener re-torquing requirements, and reduced environmental effects have been demonstrated with the new dry sealant material formulations. Furthermore, “dry” sealant materials have better abrasion resistance than the “wet” polysulfide materials; and, the lower density (approximately 1.1 gm/cm3 versus approximately 1.34 gm/cm3) results in a lower weight per unit area.
In the highly technical world of aerospace, there is an ever-present need for the significant and accurate prediction, quantification, and qualification of the various characteristics of materials such as sealants, in order to adequately compare the physical, chemical, and mechanical properties and characteristics of the sealants. For instance, although a variety of sealants are now available, there is no uniform basis available to compare the flow characteristics of one sealant material versus another. Standard measurements of viscosity do not adequately describe the flow of a cured polymer compressed between two faying surfaces. Further, standard measurements of elasticity do not adequately represent the characteristics of sealants under conditions of repeated compression and relaxation.
It is, therefore, desired to provide a method of testing sealant materials under standardized conditions that result in measurements and quantifications of the flow characteristics of these sealant materials. It is further desired to provide a method of testing the flow characteristics of these sealant materials that result in measured properties that enable the comparison of one material with another.